Such a lining is in most cases formed by overlay welding of stainless steel or nickel alloy on an inner surface of a desulfurizing reactor for petroleum-refining, which is used under a high-temperature, high-pressure hydrogen environment, in order to ensure the corrosion resistance.
During the operation under the above described environment, hydrogen is absorbed into a base metal of the vessel wall from the surface layer of the overlay welded metal (owing to the high temperature and high pressure) and is diffused through the base metal towards the outer wall surface of the vessel and it shows a stational distribution.
However, when the operation of such a reactor vessel is interrupted, that is in the case of shutdown, the separating cracks (referred to as "disbonding" hereinafter) may be caused at the welded metal side in the boundary portion of the base metal and the welded metal.
The cause of forming the disbonding has been considered as follows. Namely, when the temperature of the vessel is lowered due to the shutdown, both the hydrogen activity coefficients in the welded metal (austenite phase) and the base metal (ferrite phase) are increased but the increase in the base metal is more remarkable than that in the welded metal with respect to the temperature falling, so that the difference of both the activity coefficients is increased and hydrogen in the vessel wall reversely flows to the welded metal from the base metal and further the diffusion coefficient of hydrogen in the welded metal is smaller than that in the base metal, so that hydrogen abnormally accumulates at the welded metal side of the boundary portion of the welded metal and the base metal and this is on cause of the disbonding.
Regarding the metal structure of the boundary portion of the base metal and the welded metal, at the welded metal side, coarse austenite grains epitaxially grown with respect to the preaustenite grains at the base metal side are formed and further continuously a mixed phase of this austenite phase and the net-work formed delta-ferrite phase is formed. The above described disbonding is caused at the position corresponding to this coarse austenite grain boundary which is substantially parallel to the boundary. And, in this coarse austenite grain boundary carbides have been precipitated during the heat treatment after the overlay welding and therefore the precipitated carbides highly influence the disbonding together with the function of hydrogen accumulated during the shutdown in the operation.
Accordingly, this invention aims to provide a method for preventing the disbonding caused at the boundary between the base metal and the stainless steel overlay welded metal on an inner surface of a chemical reactor vessel in which a high-temperature, high-pressure hydrogen is handled, when the above described lining is applied.
In addition, this invention aims to provide a method for forming an overlay clad having excellent resistance to the above described disbonding and other excellent properties to be provided as the overlay welded metal, particularly high resistance to flexural elongation.